The company gave Chinese investigators information that helped them trace a personal Yahoo! email belonging to the jailed reporter Shi Tao, said the group Reporters Without Borders.

Shi, a reporter for the financial publication Contemporary Business News, was jailed for 10 years in April after being found guilty of illegally providing state secrets to foreigners.

The email for which he was jailed had notes on a government circular that spelled out restrictions on the media.

Reporters Without Borders described Shi as a "good journalist who has paid dearly for trying to get the news out".

The group added: "Does the fact that [Yahoo!] operates under Chinese law free it from all ethical considerations? How far will it go to please Beijing?"

The criticism marks the latest instance in which a prominent hi-tech firm has been accused of cooperating with Chinese authorities to gain favour. Google and MSN have been criticised for censoring online news sites and blogs with content that China wanted to suppress.